The present invention relates to a granular sorbent material reducing the SO.sub.2 content of an atmosphere at room temperature. The SO.sub.2 content is reduced by retention of the sulfur values on a MnO.sub.x -Al.sub.2 O.sub.3 solid complex. The exact nature of this retention is not fully understood, but it is apparent that it is not based on the known stoichiometry of the SO.sub.2 -MnO.sub.2 type reaction. The amount of SO.sub.2 removed is clearly greater than stoichiometry, or the sum of the stoichiometry and adsorption on alumina; hence, it could be classed as a catalytic reaction.
The MnO.sub.x -Al.sub.2 O.sub.3 complex of this invention is prepared by several methods. One method involves treating granular activated alumina with an aqueous solution of potassium permanganate, followed by drying the solid and thermally decomposing the permanganate to give a complex comprising manganese values in the oxidation states of from 2.5 to 3.5 on an alumina substrate, preferably in the form of a solid solution. We have found that the resultant granular product is especially suited for use as the active component of a respirator to effectively remove SO.sub.2 from an ambient atmosphere.
It is well known in the art that KMnO.sub.4 will react with SO.sub.2 and the prior art suggests the use of this technology to effect the conversion of noxious SO.sub.2 to a harmless or otherwise useful product. The reactions involved are: EQU 2KMnO.sub.4 + 2H.sub.2 O + 5SO.sub.2 .fwdarw.2MnSO.sub.4 + K.sub.2 SO.sub.4 + 2H.sub.2 SO.sub.4
or EQU 2KMnO.sub.4 + 2H.sub.2 O + 5SO.sub.2 .fwdarw.2MnSO.sub.4 + 2KHSO.sub.4 + H.sub.2 SO.sub.4
turk, Mehlman and Levine, Atmospheric Environment 7, pp. 1139-1148 (1973), discloses the reduction of various odoriferous compounds by use of permanganate treated carbon or alumina. The MnO.sub.2 /Al.sub.2 O.sub.3 medium utilized was prepared by reducing the permanganated alumina in an atmosphere of formaldehyde followed by heating to drive off residual formaldehyde and formic acid.
Hanna, Kuehner, Karnes and Garbowicz, Ann. N.Y. Acad. Sci. 116 (2), pp. 663-675 (1964), report a commercial air deodorant system based on a combination of potassium permanganate with activated alumina in the shape of pea-sized balls. They teach that the permanganate must be solubilized (in H.sub.2 O) to react with odor causing material since "Crystalline potassium permanganate alone will absorb neither the odors nor the moisture necessary for the oxidation reaction to take place." Any water soluble permanganate is reported to be effective with various substrates, e.g., silicas, aluminas, activated clays.
Tarbutton et al., U.S. Pat. No. 2,984,545 (5-16-61) disclose a cyclic process for removing SO.sub.2 from waste gases involving the reaction of manganese oxides with SO.sub.2 in an aqueous scrubbing system. In Example 1, the inventors state that "Manganese dioxide also was found to be inferior to the mixture of oxides."
Stephens et al., U.S. Pat. No. 3,207,704 (9-21-65) disclose catalysts for oxidation of hydrocarbons and carbon monoxide which comprise manganese oxides (Mn.sub.2 O.sub.3 and Mn.sub.3 O.sub.4 are present) on granular alumina. They teach 0.5 to 25 weight percent manganese as the oxide on a transitional alumina comprising 10-85% Chi-, 10-85% alpha-alumina monohydrate and 5-45% amorphous alumina, 0.02-5% SiO.sub.2 and a surface area of at least 75 m.sup.2 /gm.
A. B. Stiles, U.S. Pat. No. 3,230,182 (1-18-66) discloses a catalyst for conversion of hydrocarbons and carbon monoxide consisting of nickel and cobalt chromite on alumina which has manganese oxide (Mn.sub.2 O.sub.3 and Mn.sub.3 O.sub.4) as a coating on its surface. The manganese oxides are present on the alumina in the range of 1-12% by weight.
S. R. Zimmerley, U.S. Pat. No. 3,330,096 (7-11-67) discloses the use of manganese nodules for removing SO.sub.2 from polluted atmospheres. The manganese nodules contain 24.9% Mn and small amounts of Cu, Ni and Co with about 10% Fe. In addition, they contain other substances in significant amounts such as silica and alumina. It is asserted that the manganese will remove SO.sub.2 with 98% or greater efficiency even when the amount of SO.sub.2 is present in small concentrations.
Atsukawa et al., U.S. Pat. No. 3,485,014 (12-23-69) describe a cyclic dry oxidation process for removing sulfur dioxide from waste gases utilizing active manganese oxide powder.
Kawahata, U.S. Pat. No. 3,574,562 (4-13-71) discloses utilization of finely divided manganese dioxide to remove SO.sub.2 from waste gases.
Spedden et al., U.S. Pat. No. 3,723,598 (3-27-73) disclose a cyclic process for removing SO.sub.2 and SO.sub.3 from a waste effluent using manganous oxide. MnO is stated to be superior to higher valent manganese oxide.
Atsukawa et al., U.S. Pat. No. 3,798,310 (3-19-74) teach a cyclic process involving dry adsorption of SO.sub.2 on MnO.sub.1+i.XH.sub.2 O where i is 0.5 to 0.8 and X is 0.1 to 1.0. It is futher stated that the manganese oxide has a bulk density between 0.337-0.625 gm/cm.sup.3 and surface area of 125-135 m.sup.2 /gm. Various types of manganese oxides are described.
Kuowenhoven et al., U.S. Pat. No. 3,832,445 (8-27-74) describe removing SO.sub.2 from an SO.sub.2 -O.sub.2 containing gas by contacting said gas with CuO supported on particulate alumina, as well as other substrates.
Van Helden et al., U.S. Pat. No. 3,501,897 (3-24-70) also utilize solid materials to remove SO.sub.2 from waste gases. Among the solid compounds taught are copper compounds adsorbed on alumina.